Antimicrobial compounds, such as traditional antibiotics, have the ability to kill or to retard the growth of bacteria, fungi, and other microorganisms. Some antimicrobial compounds also are effective against viruses. Antimicrobial compounds are used in a wide variety of clinical settings, industrial applications, food production facilities and environmental applications all across the globe in an effort to reduce the risk of, for example, bacterial colonization and development of disease in people.
Traditional antibiotics are primarily derivatives or synthetic mimics of natural compounds secreted by bacteria, plants, or fungi. These compounds typically have very specific methods of action against a cell wall/membrane component of bacteria, or an enzyme/protein in a metabolic pathway. Examples of traditional antibiotics on the market include penicillin, oxacillin, vancomycin, gentamicin, rifampicin and amoxicillin, among others.
Because bacteria have the ability to develop resistance genes to these antibiotics as a result of genetic mutations or acquired defense mechanisms that target the specific activity of the antibiotics, bacteria typically have the ability to develop resistance to traditional antibiotics. Increasingly more prevalent bacterial resistance has made traditional antibiotics to become less and less effective in a variety of applications.
Bacterial resistance to antibiotics represents one of the most underappreciated threats to modern society. See Zhang et al., Antibiotic resistance as a global threat: Evidence from China, Kuwait and the United States, Global Health 2, 6 (2006). Currently, more than 90% of clinical isolates of Staphylococcus aureus display resistance to penicillin. See Balaban et al., Control of Biofilm Infections by Signal Manipulation, Ch. 1, 1-11 (Springer, 2008). Recent reports have even indicated that bacteria in natural ecosystems metabolize antibiotics as an energy source. See Leslie, Germs Take a Bite Out of Antibiotics, Science 320, 33 (2008). The trend of bacterial resistance continues to increase as indicated by almost daily scientific publications and world news reports of antibiotic resistant superbugs such as carbapenem-resistant Enterobacteriacea, vancomycin-resistant Enterococci, multidrug-resistant Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus (MRSA). See, e.g., FoxNews.com. Europe in the Grip of Drug-Resistant Superbugs (2011); Melnick, M., TIME (2010); Arias et al., The rise of the Enterococcus: beyond vancomycin resistance, Nat Rev Microbiol 10, 266-278 (2012); Jain, R. et al., Veterans affairs initiative to prevent methicillin-resistant Staphylococcus aureus infections, N Engl J Med 364, 1419-1430 (2011); Nordmann et al., The real threat of Klebsiella pneumoniae carbapenemase-producing bacteria, Lancet Infect Dis 9, 228-236 (2009); Aloush et al., Multidrug-resistant Pseudomonas aeruginosa: risk factors and clinical impact, Antimicrob Agents Chem 50, 43-48 (2006). In addition to adversely affecting civilian patients, antibiotic-resistant bacteria affect injured military personnel. Multiple reports from Operation Iraqi Freedom/Operation Enduring Freedom have indicated that multidrug-resistant bacteria and antibiotic resistance constitute one of the most disconcerting aspects of military theater treatment. See, e.g., Calhoun et al., Multidrug-resistant Organisms in Military Wounds from Iraq and Afghanistan, Clinical Orthopaedics and Related Research 466, 1356-1362 (2008); Murray et al., Bacteriology of War Wounds at the Time of Injury, Military Medicine 171, 826-829 (2006); Hujer et al., Analysis of Antibiotic Resistance Genes in Multidrug-Resistant Acinetobacter sp. Isolates from Military and Civilian Patients Treated at the Walter Reed Army Medical Center, Antmicrobial Agents and Chemotherapy 50, 4114-4123 (2006).
Multiple factors contribute to bacterial cells' ability to resist the effects of antibiotics. See, e.g., Morita et al., Antibiotic Inducibility of the MexXY Multidrug Efflux System of Pseudomonas aeruginosa: Involvement of the Antibiotic-Inducible PA5471 Gene Product, Journal of Bacteriology 188, 1847-1855 (2006); Tran et al., Heat-Shock Protein ClpL/HSP 100 Increases Penicillin Tolerance in Streptococcus pneumoniae, Advances in Oto-rhino-laryngology 72, 126-128 (2011); Livorsi et al., Virulence Factors of Gram-Negative Bacteria in Sepsis With a Focus on Neisseria meningitidis, Contributions to Microbiology 17, 31-47 (2011); Nostro, et al., Specific Ion Effects on the Growth Rates of Staphylococus aureus and Pseudomonas aeruginosa, Physical Biology 2, 1-7 (2005). Amongst these factors is the ability of bacteria to develop a biofilm. See, e.g., Costerton et al., How bacteria stick, Sci Am 238, 86-95 (1978); Lawrence et al., Optical sectioning of microbial biofilms, J Bacteriol 173, 6558-6567 (1991); ZoBell, The Effect of Solid Surfaces upon Bacterial Activity, Journal of Bacteriology 46, 39-56 (1943). Biofilms have unique characteristics that allow them to withstand, or defend themselves against a variety of perturbations including exposure to antibiotics.
Biofilms are surface-attached communities of bacteria, often polymicrobial, that produce a slimy, extracellular polysaccharide substance (EPS) that encapsulates them. The EPS provides protection, Leid et al., The Exopolysacharide Alginate Protects Pseudomonas aeruginosa Biofilm Bacteria from IFN-γ-Mediated Macrophage Killing, The Journal of Immunology 175, 7512-7518 (2005), as well as a reserve of nutrients, water and trace elements to sustain life. Costerton et al., The Bacterial Glycocalyx in Nature and Disease, Annual Review of Microbiology 35, 299-324 (1981). Biofilms are the predominant phenotype of bacteria in natural ecosystems. Gram-negative bacteria, Gram-positive bacteria, and mycobacteria, in addition to other unicellular organisms, can produce biofilms.
Within the biofilm community, bacteria may have several methods of defending themselves against the biocidal effects of antibiotics. First, they have strength in numbers. Biofilms may contain millions or trillions of cells in a very small volume. Second, bacteria in a biofilm have the ability to rapidly transfer genetic material, such as plasmids, that specifically code for the production of molecules that protect them against antibiotics. Lujan et al., Disrupting Antibiotic Resistance Propagation by Inhibiting the Conjugative DNA Relaxase, PNAS 104, 12282-12287 (2007); Lederberg et al., Gene Recombination in Escherichia coli. Nature 158, 529-564 (1946). Rates of plasmid transfer in biofilms have been shown to be much higher than amongst planktonic bacteria, which are free-floating in an environment. Hausner et al., High Rates of Conjugation in Bacterial Biofilms as Determined by Quantitative In Situ Analysis, Applied and Environmental Microbiology 65, 3710-3713 (1999). Third, as a biofilm community matures, it creates an oxygen gradient such that an oxygen-rich environment exists on the outer edges of a biofilm, whereas an oxygen-deprived, or anaerobic, area exists in the deepest portions of a biofilm. Walters et al., Contributions of Antibiotic Penetration, Oxygen Limitation, and Low Metabolic Activity to Tolerance of Pseudomonas aeruginosa biofilms to Ciprofloxacin and Tobramycin, Antimicrobial Agents and Chemotherapy 47, 317-323 (2003); Borriello et al., Oxygen Limitation Contributes to Antibiotic Tolerance of Pseudomonas aeruginosa in Biofilms, Antimicrobial Agents and Chemotherapy 48, 2659-2664 (2004). This may result in reduced metabolic activity in those cells that dwell in the interior of the biofilm. Importantly, traditional antibiotics are typically effective against bacterial cells that are rapidly dividing, i.e., in a logarithmic phase of growth. Mandell, Interaction of Intraleukocytic Bacteria and Antibiotics, The Journal of Clinical Investigation 52, 1673-1673 (1973); Gilbert et al., Influence of Growth Rate on Susceptibility to Antimicrobial Agents: Biofilms, Cell Cycle, Dormancy, and Stringent Response, Antimicrobial Agents and Chemotherapy 34, 1865-1868 (1990). Fourth, in a mature biofilm, water channels form throughout the community. Stoodley et al., Liquid flow in biofilm systems, App Env Microbiol 60, 2711-2716 (1994). These water channels have the ability to diffuse, remove or prevent toxic byproducts as well as antibiotics from interacting with cells in the biofilm. For novel antimicrobial agents to be effective over the long term, addressing each of these four characteristics may increase the potential for success in a variety of applications including healthcare, industrial, environmental, agricultural and sanitation industries. Furthermore, biofilms tend to secrete proteoglycan materials that create an extracellular matrix, which has the ability to potentially bind and hinder the activity of antibiotics.
Alternative approaches to killing bacteria include the use of antimicrobial agents that have fast-acting and nonspecific mode of activity against the cell membrane of bacteria. These alternate compounds include detergents, squalamine, quaternary ammonium compounds, and naturally occurring antimicrobial peptides, among others. By attacking and depolarizing the cell membrane in a nonspecific fashion at a faster rate, agents that attack the cell membrane globally can kill bacteria before they have time to upregulate their defense mechanisms. In addition, modes of action of these alternate antimicrobials are not limited to a specific protein or enzyme within a metabolic pathway.
However, as important as it is to kill bacteria and prevent their ability to cause infections in humans or animals, or contaminate unwanted processes in industrial, agricultural or environmental applications, when bacteria are attached to a surface, it sometimes may be more beneficial to not only kill bacteria, but also to cause them to “fall off” of a surface as well, e.g. disperse or dislodge bacteria in a biofilm community. In certain aspects, the present invention provides compounds, compositions, and methods that have shown the ability to disperse or dislodge bacterial cells in a biofilm, such that the cells are no longer able to reattach and form new biofilm communities, and, notably, the same compounds, compositions, and methods kill substantially all bacteria cells in a biofilm.
By dispersing a biofilm and killing the cells within it, at least two benefits are provided. This may be particularly important when considering the fact that although bacteria in a biofilm, which may be attached to a surface, can be killed by an antimicrobial agent, the dead cells and extracellular matrix residues may provide an attachment point for viable bacteria to re-adhere and form a biofilm once again with greater affinity. If biofilms are dispersed and killed, viable bacteria that are introduced to a surface will have reduced ability to preferentially adhere to that area. This can be particularly important in industrial applications wherein the formation of biofilms on a surface can be problematic, as well as medical applications wherein bacteria may adhere to the surface of a medical device. It has been surprisingly discovered that compositions of the present invention have significant potential to eradicate bacteria within a biofilm as well as cause the biofilm to disperse or dislodge, resulting in a variety of potential applications across multiple settings.
Thus, there is a need for novel compounds, compositions, and methods that have potent antimicrobial and anti-biofilm activity against a variety of bacterial strains, especially at high bacterial concentrations and against antibiotic-resistant bacteria.